KVM: Don't make a full copy of the old memslot in __kvm_set_memory_region()

Stop making a full copy of the old memslot in __kvm_set_memory_region()
now that metadata updates are handled by kvm_set_memslot(), i.e. now that
the old memslot's dirty bitmap doesn't need to be referenced after the
memslot and its pointer is modified/invalidated by kvm_set_memslot().

No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Message-Id: <5dce0946b41bba8c83f6e3424c6955c56bcc9f86.1638817640.git.maciej.szmigiero@oracle.com>

virt/kvm/kvm_main.c

@@ -1730,8 +1730,8 @@ static int kvm_set_memslot(struct kvm *kvm,
 int __kvm_set_memory_region(struct kvm *kvm,
 			    const struct kvm_userspace_memory_region *mem)
 {
-	struct kvm_memory_slot old, new;
-	struct kvm_memory_slot *tmp;
+	struct kvm_memory_slot *old, *tmp;
+	struct kvm_memory_slot new;
 	enum kvm_mr_change change;
 	int as_id, id;
 	int r;
@@ -1761,25 +1761,16 @@ int __kvm_set_memory_region(struct kvm *kvm,
 		return -EINVAL;
 
 	/*
-	 * Make a full copy of the old memslot, the pointer will become stale
-	 * when the memslots are re-sorted by update_memslots(), and the old
-	 * memslot needs to be referenced after calling update_memslots(), e.g.
-	 * to free its resources and for arch specific behavior.
+	 * Note, the old memslot (and the pointer itself!) may be invalidated
+	 * and/or destroyed by kvm_set_memslot().
 	 */
-	tmp = id_to_memslot(__kvm_memslots(kvm, as_id), id);
-	if (tmp) {
-		old = *tmp;
-		tmp = NULL;
-	} else {
-		memset(&old, 0, sizeof(old));
-		old.id = id;
-	}
+	old = id_to_memslot(__kvm_memslots(kvm, as_id), id);
 
 	if (!mem->memory_size) {
-		if (!old.npages)
+		if (!old || !old->npages)
 			return -EINVAL;
 
-		if (WARN_ON_ONCE(kvm->nr_memslot_pages < old.npages))
+		if (WARN_ON_ONCE(kvm->nr_memslot_pages < old->npages))
 			return -EIO;
 
 		memset(&new, 0, sizeof(new));
@@ -1799,7 +1790,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
 	if (new.npages > KVM_MEM_MAX_NR_PAGES)
 		return -EINVAL;
 
-	if (!old.npages) {
+	if (!old || !old->npages) {
 		change = KVM_MR_CREATE;
 
 		/*
@@ -1809,14 +1800,14 @@ int __kvm_set_memory_region(struct kvm *kvm,
 		if ((kvm->nr_memslot_pages + new.npages) < kvm->nr_memslot_pages)
 			return -EINVAL;
 	} else { /* Modify an existing slot. */
-		if ((new.userspace_addr != old.userspace_addr) ||
-		    (new.npages != old.npages) ||
-		    ((new.flags ^ old.flags) & KVM_MEM_READONLY))
+		if ((new.userspace_addr != old->userspace_addr) ||
+		    (new.npages != old->npages) ||
+		    ((new.flags ^ old->flags) & KVM_MEM_READONLY))
 			return -EINVAL;
 
-		if (new.base_gfn != old.base_gfn)
+		if (new.base_gfn != old->base_gfn)
 			change = KVM_MR_MOVE;
-		else if (new.flags != old.flags)
+		else if (new.flags != old->flags)
 			change = KVM_MR_FLAGS_ONLY;
 		else /* Nothing to change. */
 			return 0;